In web printing and web coating apparatus, provision must often be made for contactless support of a substantially long stretch of web that directly follows a printing or coating station, to prevent smearing or other damage to the freshly applied printing ink or coating material. For such contactless support the web is floated on a cushion of pressurized air. Usually there are one or more locations at which the air supported portion of the web must be guided through a change of direction, and at each such location the web is controlled for its change of direction by means of a contactless turning guide, sometimes referred to in the art as an "air turn."
As in the structure disclosed in U.S. Pat. No. 4,182,472, which can serve by way of example, a turning guide has a curved exterior surface around which the web is guided and has a hollow interior that comprises a plenum chamber into which pressurized supply air is fed. The pressure air issues from the plenum chamber through slot-like outlets that open to the curved guide surface, to maintain a cushion of pressure air between the web and said surface. The distance between the guide surface and the web--which can be designated the web clearance--is normally on the order of 1/16 to 1/8 inch.
For a given web tension, the web clearance depends upon the pressure and rate of flow of the pressurized supply air fed to the plenum chamber, increasing with increasing pressures and flow rates. On the other hand, if web tension is relaxed while the flow of air into the plenum chamber is maintained constant, the web tends to balloon away from the turning guide surface and flutter, becoming laterally uncontrollable; whereas if web tension is substantially increased without an increase in supply air, the web may drag against the turning guide.
Usually the pressure air source is a fan or blower, and a damper controls the rate at which air flows from the fan into the turning guide plenum chamber. Normally, a fixed setting of the damper is adequate for a given set of running conditions, but it is almost always necessary to change the damper setting during start-up, stopping or shut-down because of substantial changes in web tension at those times. The need for manually controlling or effecting changes in damper setting imposes a heavy work load on operating personnel at times when they are busiest, and therefore there has been a long-standing need for automatic means for controlling the position of the supply air damper in accordance with web tension, in order to maintain a substantially constant web clearance.
Several types of devices have been proposed for effecting automatic control of damper setting. Heretofore it has been thought that a basic element necessary for such automatic control was a sensor for detecting the distance between the web and the curved turning guide surface and for producing a damper-controlling output corresponding to a function of that distance.
Consideration has been given to several types of distance sensing devices for this purpose, but all of them are significantly objectionable in one way or another. High frequency electrical eddy current devices are generally satisfactory for measuring small distances, but such devices function only with electrically conducting materials and are therefore useless with most paper and plastic webs. Ultrasonic devices have been used for distance measurement on some automatically focusing cameras, and have been employed for accurate measurement of small distances on metal parts, but when an ultrasonic signal has to traverse air it does not afford the resolution needed for accurate measurement of such small distances as must be maintained between a web and a turning guide surface. A pneumatic nozzle for sensing back pressure would have to open towards the surface of the web remote from the turning guide in order not to be affected by cushion air pressure variations as well as by web clearance changes; and although such a nozzle could produce an output related to web clearance distance, it would have to be so close to the web, in order to have the required sensitivity, that the web would be likely to drag against it with any substantial decrease in web tension, and the nozzle would in any case be in the way during web threading. Most optical distance sensing devices would not be satisfactorily operative under all conditions; as, for example an optical distance sensor responsive to reflectivity of the web would be confused by a printed web. An optical sensor operating on the basis of reflection geometry such as angle of reflection, although theoretically satisfactory, would be unduly expensive.
The present invention takes a fundamentally different and unobvious approach to the problem, in that it rests upon the premise that a proximity sensor or distance measuring device is unnecessary for maintenance of a constant distance between the web and the curved surface of a turning guide. Instead, the present invention provides a method and means for utilizing a relationship that is inherent in the properties of the turning guide itself to accomplish automatic control of the supply of pressure air to the plenum chamber in the turning guide for maintenance of a substantially constant web clearance.